Process for production of synthesis gas in combination with the maintenance of the energy balance for a pulp mill

ABSTRACT

A process ( 44 ) for the production of pulp and paper ( 28 ), recycling of cooking chemicals ( 3, 29 ), combustion of biomass ( 30, 37 ) and generation of heat and electric energy ( 27, 40 ) comprising a pulp and paper mill ( 28 ), in that the part of the process which is recycling cooking chemicals is adjusted from combustion ( 29 ) to gasification ( 3 ) to generate synthesis gas ( 14 ); and that biomass is added ( 33, 39 ) in an amount sufficient for compensating of the decrease in heat and electricity generation as a consequence of the generation of synthesis gas ( 14 ).

TECHNICAL FIELD

[0001] The present invention relates generally to the field of efficientproduction of synthesis gas for further conversion to products such asmethanol, DME, hydrogen gas or other valuable chemicals, from biomassderived material and in particular to synthesis gas production incombination with the production of pulp and paper. More specifically,the present invention relates to the chemical recovery process and theoverall energy balance for a pulp mill.

BACKGROUND OF THE INVENTION

[0002] The strong dependence on fossil fuels, more specifically crudeoil, in the energy and transport sector where disturbances in the supplyhave great impact on the economies throughout the world has lead toincreased activities in the search for alternative sources for energy.

[0003] The increased usage of fossil fuels leads to an increased levelof carbon dioxide in the atmosphere, which is strongly believed to havean impact on the global climate. The so-called greenhouse effect iscaused by the presence of certain gases such as carbon dioxide, watervapour and methane in the atmosphere. An increased concentration ofthese gas components in the atmosphere may have an impact of thetemperature level and lead to globally warmer climate. Due to thecontinued and increased usage of fossil fuels the concentration ofcarbon dioxide increases steadily in the earth's atmosphere withpossible severe consequences for economies and basis for life.

[0004] The search for reliable and ecologically sustainable solutions tothe world's energy demands have been on-going since the first oil crisisin 1974. Conversion of renewable, biomass-based energy sources toelectric power and to automotive fuels has however shown to betechnically difficult and expensive and only a few real demonstrationshave been realised.

[0005] The chemical pulping of wood and other lignocellulosic materialsis a well-established process to produce pulp and paper products. Themost common process is the kraft pulping process where sulphur-andsodium-based chemicals are used when digesting the wood chips into pulp.The invention is also applicable to other chemical pulping processessuch as sodium carbonate based non-sulphur processes. At the outlet ofthe pulp mill's digester step the pulp is separated from the cookingchemicals and dissolved wood constituents of which the lignin is themajor part. This separated stream is concentrated in a multi-effectevaporator system to a dryness of 65-85% and is called black liquor.This intermediate stream in a kraft mill is the key energy carrier andprovides the major part of the energy required by the kraft processwhich is a large consumer of electric power and heat.

[0006] State-of-the-art technology in a kraft mill for recovery of theenergy and the chemicals from the black liquor is to feed it to arecovery boiler, a so-called Tomlinson boiler, where the inorganiccooking chemicals are recovered as a smelt at the bottom of the boilerand withdrawn for recycling to the process and the organic material iscombusted and the heat recovered as usable energy by generation ofsteam.

Existing Technoloay of Today—Recovery Boiler Technology

[0007] The chemical and energy recovery system for a state-of-the-artkraft mill is further described with reference to FIG. 2. The thicknessof the streams in this figure as well as in FIGS. 3-6 indicates relativequantities of energy bound to the streams in the various processes.

[0008] Pulp wood (19) is brought into the mill and is freed from barkbefore being chopped Into wood chips for further processing. The barkstream (20), is fed to a biomass fired power boiler (30). In the millprocess (28) the wood chips are converted into pulp (if the plant onlyproduces pulp it is a so-called non-integrated mill) or into paper (ifthe plant is a combined pulp and paper mill, a so-called integratedmill) (22). The non-pulp elements of the wood together with the cookingchemicals together form a thin black liquor which is concentrated in anevaporation plant to a dryness of 65-85% and called black liquor (23)and is then fed to the recovery boiler (29). In the recovery boiler (29)the cooking chemicals are separated and thereafter recycled to the millprocess in the form of so-called green liquor (24) at the same time asthe energy in the black liquor is converted to steam (25). Lowtemperature non-recoverable energy leaves the system through therecovery boiler stack (31).

[0009] To create balance between energy demand and supply for the millprocess, steam is brought to the process via stream (27). The energyrequired comes from the recovery boiler (29) through stream (25) andfrom the power boiler (30) through stream (26). To keep the energy inthe entire system in balance, an extra amount of biomass needs to bebrought to the power boiler (30) on top of what is brought there interms of bark (20) coming from the wood (19). This stream is shown asstream (21). The total need of biomass derived feedstock to the mill inthe form of wood for pulping and biomass for energy generation istherefore the sum of streams (19), (20) and (21). If the mill isnon-integrated, the bark (20) is often sufficient to make up for theenergy balance.

Technology for the Replacement of the Recovery Boiler

[0010] Over the past 25 years there have been a number of developmentsgoing on to improve the energy recovery of the kraft pulp process bymoving from the current recovery boiler based technology to a conceptinvolving a pressurised gasification reactor. The black liquor stream isthus only partially oxidized or gasified to a combustible gas instead ofbeing completely burnt. Such a concept is e.g. described in thepublication by Berglin et al, 2^(nd) Biennal Johan Gullichsen Colloqium,Helsinki, Finland, Sep. 9-10, 1999, and a preferred embodimentdescribing the gasification reactor configuration is described in U.S.Pat. No. 4,808,264. These two documents are included as references. Thesystem is commonly referred to as a BLGCC system, an abbreviation forBlack Liquor Gasification Combined Cycle.

[0011] The BLGCC system combines the pressurised gasification withfiring of the combustible gas in a gas turbine, which in turn iscombined with a waste heat boiler and a steam turbine togethercomprising a so-called combined cycle (CC). The inclusion of a BLGCCsystem in a pulp mill increases the overall energy yield by about 10percentage points at the same time as the yield converted to electricpower almost doubles compared with the performance of a modem recoveryboiler.

[0012] The chemical and.energy recovery system of a mill including aBLGCC system is further described with reference to FIG. 3.

[0013] Pulp wood (19) is brought into the mill and is freed from barkbefore being chopped into wood chips for further processing. The bark isfed to a biomass fired power boiler (30). In the mill process (28) thewood chips are converted to pulp and paper (22). The non-pulp elementsof the wood together with the cooking chemicals together form a thinblack liquor which is concentrated to a dryness of 65-85% and calledblack liquor and then fed to an evaporation plant and then fed to theBLGCC system (32). The gasification process within the BLGCC systemseparates and recycles the cooking chemicals in the form of so calledgreen liquor (24) to the mill process (28). In the BLGCC process, thesulphur in the gas is separated out and brought back to the mill processin stream (35) before the clean gas is fed to the gas turbine. The hotexhaust gas from the gas turbine is utilised to produce high-pressuresuperheated steam, which is fed to a steam turbine before the cooledexhaust stream is emitted to the atmosphere through stream (36).

[0014] As mentioned above the total production of power from a millincorporating a BLGCC system is close to twice as high as for thecorresponding recovery boiler solution as per FIG. 2 and the BLGCC basedconcept will be a net exporter of power. Power is exported throughstream (34). Due to the higher power production there is a lower steamsupply from the BLGCC system back to the mill in stream (25) comparedwith the recovery boiler case. The requirement of energy (27) to themill (28) is however the same as for a mill combined with a recoveryboiler (29) and therefore the production of steam in stream (26) fromthe power boiler (30) must be increased by the corresponding amount.Additional biomass must therefore be brought to the power boiler (30) instream (33).

[0015] The total need of biomass derived feedstock in the form of woodfor pulping and biomass for energy generation for a mill having a BLGCCsystem is therefore the sum of streams (19), (20),(21) and (33) wherethe three first streams are identical to the case for a mill combinedwith a recovery boiler.

A State-of-the-Art Technology for Methanol Production

[0016] DE-A1-1517207 discloses a process for methanol production byblack liquor gasification.

[0017] Commercial methanol production is based on synthesis gas producedby gasification of heavy oil, coal and natural gas.

[0018] Conversion of renewable feedstocks such as lignocellulosic typesof biomass to methanol has been investigated in a large number ofstudies since the early 1980's. FIG. 1 shows a biomass to methanolproduction plant comprising the following process steps: Biomassfeedstock drying and handling (1), air separation to produce pure oxygen(2), pressurised gasification with oxygen to produce a synthesis gas(3), synthesis gas cooling (4) , synthesis gas purification (5),synthesis gas conditioning (6), methanol synthesis (7). All listedprocess steps are well established except for the conversion of biomassthrough gasification with oxygen which has been tested only in pilotscale and during short periods.

[0019] The conversion of biomass to methanol can be carried outaccording to two main principles which the first can be designated as amethanol only route, concept A, and the second as a methanol plusby-product route, concept B.

[0020] Concept A is illustrated with the seven process steps shown inFIG. 1. The conversion efficiency from biomass to methanol is, withreference to above studies, approximately 50% and may reach a fewpercentage points higher when further optimised.

[0021] Pressurised gasification of solid biomass (3) has somechallenging features which need further development to reach commercialstatus and that may turn out to be severe obstacles to the realisationof the total concept.

[0022] The introduction of biomass under pressure (9) requires a specialfeeding system working with a pressurising gas (17) which must be ascompatible as possible for the downstream methanol synthesis (7) inorder to minimise the size of the bleed out stream (16) from themethanol synthesis loop (7).

[0023] Gasification of solid biomass is normally carried out in afluidised bed type of reactor with chopped biomass with a meancharacteristic particle size of 5-50 mm. (Fixed bed reactors arenormally limited to only 5 MWth/unit due to heat transfer.) Entrainedflow reactors require a smoothly conveyable feedstock and woodybiomasses are generally not pneumatically conveyable nor pumpable. Screwfeeding is normally a preferred conveying method which can be used influidised beds using a protecting and inertisation gas that prevents hotmaterial and gas from the reactor to enter the conveyor mechanism thatcan cause plugging.

[0024] Biomass generally have a low ash content (of about 1% weight) andwhen the ash contains enough alkali metals to can cause bed materialagglomeration to cause plugging of the reactor may result or lead tofouling problems. The fluidised bed sand material must therefore bereplaced. Loss of bed material also occurs by attrition (particleerosion) and elutriation (loss of fine part of particle fraction via thecyclone gas stream.) Spent bed materials may have to be safely depositedas they can contain leachable hazardous components.

[0025] For the reason of the bed material agglomeration risk describedabove, the reactor temperature is limited to around 900° C., which is amoderate temperature that often results in the formation of undesirableby-product tars. The formed tars undergo secondary cracking producingmethane and olefins, which are generally not stable at 900° C. but areproduced by radical tar cracking. As long as tar components are presentin the gas they will be accompanied by a quantity of methane which isquite higher than the equilibrium level.

[0026] The gasification reactions in the gasifier (3) thus producemethane and other higher hydrocarbons in the raw synthesis gas (11).Methane constitutes a large share of the energy content of said raw gasand in concept A, the methane needs to be converted to synthesis gas andfurther into methanol. This conversion is a known process step (6) butmeans that more processing steps must be added leading to a more complexconfiguration of concept A.

[0027] The ratio between the two synthesis gas molecules carbon monoxideand hydrogen needs to be adjusted in order to maximise methanolproduction. This is also accomplished in the gas conditioning step (6)and interferes negatively with the methane conversion to synthesis gas.

[0028] The conditioning step (6) requires, a feed gas (13) low in carbondioxide in order to supply the methanol synthesis (7) with an optimumgas composition for maximum methanol yield.

[0029] The formation of higher hydrocarbons needs to be minimised or amethod for their elimination or capture needs to be included In theplant concept.

[0030] An alterative route here, concept B, would overcome the abovelisted difficulties with concept A. Concept B can be described as amethanol and by-product route, which simplifies the processconfiguration.

[0031] The methanol and by-product route can be described using FIG. 1with the following changes compared to concept A. The gas conditioningstep (6) is eliminated which means that no conversion of methane intosynthesis gas is carried out and preferably no adjustment of the ratiobetween carbon monoxide and hydrogen is made. As a consequence the needfor bleed out of gas (16) from the methanol synthesis (7) loopincreases.

[0032] The conversion of biomass to methanol in concept B isapproximately 25% and the total energy yield of methanol plus bleed outgas is approximately 60%. This means that the yield of methanol has comedown to about half compared to concept A but the total yield ofproducts, methanol and energy rich gas, have increased.

[0033] With a liquid feedstock such as black liquor, the pressursationstep of the gasifier feedstock stream can be performed by a simple pumpinstead of a complex system of lock hoppers that are necessary for solidfuel feeding systems according to prior art processes and systems forproduction of e.g. methanol. The burner system of the gasifier reactorcan also be simplified when fed with a pumpable liquid instead of asolid feedstock.

[0034] Depending on the properties of feedstock the gasifier reactorprinciple can also be altered to optimise the conversion of the fuel tosynthesis gas. As a consequence the methane concentration drasticallydecreases to a level which can be accepted without further treatment asdescribed earlier for concept A.

[0035] The generation of higher hydrocarbons is also suppressed due tothe favorable conditions for synthesis gas generation In the case of ablack liquor gasifier.

[0036] It is well known from many methanol production schemes where thefeedstock is a sulphur-rich heavy oil fraction or coal that the gascleaning step must be of advanced nature in orderto protect thesensitive methanol catalyst In the methanol reactor from poisoning anddegradation. The synthesis gas from the black liquor gasification stepcontains sulphur components in the form of hydrogen sulphide andcarbonyl sulphide and it also contains carbon dioxide, traces of higherhydrocarbons and possibly other traces which can be harmful for themethanol synthesis step. Technology suited to meet the high qualitydemand for methanol synthesis gas is available and commercially proventechnology. Such gas purification processes generate by-product streamswell suited to be integrated into the mill process with the potential toenhance the yield from and the performance of the kraft process. Onesuch integration benefit is described in EP-B1-0903436.

SUMMARY OF THE INVENTION

[0037] It is the objective of the present invention to create a newcombination of processes that can produce synthesis gas from biomassderived fuels in a simpler and in a more energy efficient way than whatstate-of-the-art technology has the potential to do. Preferably, thebiomass fuels can be different types of low quality such as forestalwaste, refuse derived fuels, bark or similar.

[0038] To overcome the current problems which the state-of-the-artbiomass-to-methanol technology there is need of process development aspreviously described. An alternative way to overcome such problems wouldbe to make an alteration of feedstock such that the biomass gasificationstep would produce a gas more suitable for methanol production. Thekraft pulping process in this aspect offers a unique combination offeatures as it is optimised to withdraw a maximum amount of the woodfibres for paper pulp production at the same time as it produces abiomass derived, energy rich stream in liquidi state, so-called blackliquor.

[0039] The combination of the features of the kraft pulping process withits intermediate, energy rich black liquor stream, the specialrequirement of the synthesis gas when producing methanol and thepresence of a large biomass fired boiler at the mill site, offers a highpotential of energy conservation which allows the methanol to beproduced from biomass in an exceptionally energy efficient manner. Atthe same time, the combination results in other positive synergy for thepulp mill with potential for increased pulp yield from the kraft pulpprocess.

[0040] The black liquor gasification as well as the synthesis stepsaccording to the process of the present invention are carried out duringpressurised conditions. The black liquor gasification is suitablycarried out at a pressure around or above 20-25 bar, since a lowerpressure results in an exergy loss of the recoverable heat which isevolved in the gasifier reactor. On the other hand, technical conditionsconstrains the upper limit for the pressure. The synthesis step, such ase.g. methanol synthesis, is preferably carried out in the range fromabout 60 bar up to about 80 bar.

[0041] The process according to the present invention presents asolution to the above mentioned problems by providing a process for theproduction of pulp and paper, recycling of cooking chemicals, combustionof biomass and generation of heat and electric energy comprising a pulpand paper mill, in that the part of the process which is recyclingcooking chemicals is adjusted from combustion to gasification togenerate synthesis gas; and that biomass is added in an amountsufficient for compensating for the decrease in heat and electricitygeneration as a consequence of the generation of synthesis gas.

[0042] The process of the present invention is particularly advantageousfor production of synthesis gas, preferably for further processing intoproducts such as methanol, DME, hydrogen gas or other valuable chemicalsand/or automotive fuels. The invention particularly relates suitably tothe conversion of lower quality biomass derived feedstocks. The processfacility related to the invention is conveniently to be physicallylocated close to a kraft mill facility producing chemical pulp forpapermaking.

[0043] In today's existing integrated pulp mills there is a deficit ofenergy in form of heat and electricity and the required extra energypurchased to the mill is often bark, oil or natural gas for boilerfiring and electric power from the electric grid. With the processaccording to the present invention, the energy deficit will increase dueto the withdrawal of a new energy rich product stream from the system.The deficit of electric power and heat will be met by feeding additionalbiomass derived energy material to the system. With this combination ofprocesses, lower grade energy resources, like forestry waste wood, canbe upgraded to high quality energy products like methanol, DME orhydrogen through efficient energy conservation, preferably within thesystem.

[0044] Preferably, the process according to the present inventioncomprises a production of synthesis gas for further processing intomethanol, DME, hydrogen or other valuable chemicals from biomass derivedmaterial.

[0045] According to a preferred embodiment the process of the presentinvention relates to a process wherein said synthesis gas is convertedto methanol, comprising combinations of the following processes, inwhich the second process below is not directly involved:

[0046] A first process for conversion of wood to produce pulp utilisingcooking chemicals containing sodium and sulphur based salts and alsoco-producing a biomass derived, energy rich stream containing spentcooking chemicals;

[0047] A second process for conversion of the energy in said stream tousable energy for the first process and recycle of said cookingchemicals to the first process;

[0048] A third process for conversion of the energy in said stream tomethanol and usable energy for the first process and recycle of saidspent cooking chemicals to the first process;

[0049] A fourth process for conversion of biomass derived material toheat and electric energy;

[0050] A fifth process for conversion of biomass derived material toelectric energy;

[0051] where in an original configuration comprising the first, secondand fourth processes, the energy required for operation of said originalconfiguration is partly brought to the configuration from the secondprocess where said energy rich stream from the first process isconverted to heat and electric energy and partly by conversion of saidbiomass derived material brought to the fourth process where saidmaterial is converted to heat and electric energy for said originalconfiguration, in that, when the second process is replaced by the thirdprocess to become an alternate configuration and energy in said spentcooking chemicals from the first process therefore in part is convertedto methanol and withdrawn from said alternate configuration, additionalbiomass derived energy is brought to said alternate configuration afterbeing converted to heat and electric energy in the fourth process and toelectric energy in the fifth process so that the total requirement ofheat and electric energy for said alternate configuration is met at anequal level as in said original configuration.

[0052] According to yet an embodiment, the energy withdrawn from saidalternate configuration in the form of methanol corresponds to at least60% of the energy contained in said additional biomass brought to saidalternate configuration to compensate for the withdrawal of saidmethanol.

[0053] Suitably, according to yet a further embodiment, sulphurcomponents, such as sulphide and other sulphur components, are removedfrom the syntheses gas, preferably to a concentration below about 0.1ppm, and recycled to the mill process in a highly concentrated stream.

[0054] In another embodiment, the chemical produced from, the thirdprocess can instead be DME (Di methyl ether) which is produced in aprocess closely similar to the methanol, synthesis but with a differentcatalyst and just slightly different process conditions. In such a case,the energy withdrawn from said alternate configuration in the form ofDME production suitably corresponds to atleast 60% of the energycontained in said additional biomass brought to said alternateconfiguration to compensate for the withdrawal of said DME.

[0055] As an alternative, the chemical produced from the third processcan instead of methanol or DME also be hydrogen of high purity. In sucha case, the energy withdrawn from said alternate configuration in theform of hydrogen production corresponds suitably to at least 60% of theenergy contained in said additional biomass brought to said alternateconfiguration to compensate for the withdrawal of said hydrogen.

[0056] Conveniently, the fifth process for conversion of biomass derivedmaterial to electric energy can be located at a remote location fromsaid alternate configuration and that said electric energy is brought tothe alternate configuration via an electric distribution grid.

[0057] The process of the present invention can be applied in any pulpand paper process, but is preferably carried out with the production ofpulp and paper through the kraft pulping process.

[0058] The invention is abbreviated BLGSF which stands for Black LiquorGasification with synthetic fuels generation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Preferred embodiments of thepresent invention will now bedescribed by way of example, with reference to the attached drawings, byno way restricting the present invention thereto, wherein

[0060]FIG. 1 illustrates in a diagram the state-of-the-art technologyfor biomass conversion to methanol.

[0061]FIG. 2 illustrates in a flow diagram the state-of-the-art pulp andpaper mill technology.

[0062]FIG. 3 illustrates in a flow diagram a pulp and paper millincluding BLGCC technology for black liquor conversion.

[0063]FIG. 4 illustrates in a flow diagram a pulp and paper millincluding BLGSF technology for black liquor conversion.

[0064]FIG. 5 illustrates in a flow diagram energy flow around astate-of-the-art pulp and paper mill.

[0065]FIG. 6 illustrates in a flow diagram the energy flow around a pulpand paper mill integrated with methanol production from renewables.

DETAILED DESCRIPTION OF THE INVENTION

[0066] As already discussed above in the background of the invention,the state-of-the-art technology for biomass conversion to methanol isshown in FIG. 1, today's technology for black liquor conversion is shownin FIG. 2, and the BLGCC technology for black liquor conversion is shownin FIG. 3.

[0067] The chemical and energy recovery system combined with methanolproduction is described with reference to FIG. 4. The methanolproduction process 38 is identical with FIG. 1 with feedstock dryingstep 1 eliminated.

[0068] Pulp wood 19 is brought into the mill and is freed from barkbefore being chopped into wood chips for further processing. The bark isfed to a biomass fired power boiler 30. In the mill process 28, the woodchips are converted to pulp and paper 22. The non-pulp elements of thewood together with the cooking chemicals together form a thin blackliquor which is concentrated in an evaporation plant and then fed to theBLGSF system 38. The gasification process within the BLGSF system 3,separates and recycles the used cooking chemicals in the form ofso-called green liquor 24 to the mill process 28. The methanolproduction process 38 requires steam and power to produce the methanolproduct, stream 15. The conversion efficiency of synthesis gas tomethanol is high, which results in that less heat can be recycled backto the mill process in stream 25 in comparison with the state-of-the-artconfiguration, FIG. 2., where a recovery boiler 29 is utilised for heatrecovery.

[0069] The requirement of energy 27 to the mill 28 is however the sameas for a mill combined with a recovery boiler and therefore theproduction of steam 26 from the power boiler 30 must be increased tocompensate for the lower amount of heat in stream 25. Additional biomassis therefore brought to the power boiler 30 in stream 33.

[0070] In comparison to the two other presented cases as per FIGS. 2 and3, a mill combined with a BLGSF process will need additional electricpower generation in order to reach the same degree of independence ofimport of fuel and power supply from its surroundings as for outside thetwo other cases. This is accomplished via the use of a biomass fedgasification plant combined with a so-called combined cycle operated inthe condensing mode 37. The technology is commonly abbreviated biomassfed IGCC which stands for Integrated Gasification Combined Cycle andwhich is used to maximise the electric power efficiency.

[0071] In FIG. 4 the biomass needed for the extra power generation isfed to the biomass fed IGCC unit 37 through stream 39 together withbleed-out of purge gas 16 from the methanol synthesis step. The electricpower is fed to the processes through stream 40.

[0072] The overall need of biomass derived feedstock, in the form ofwood for pulping and biomass for energy generation for a millincorporating a BLGSF system, is therefore the sum of streams 19, 20,21, 33 and 39 where the three first streams are identical to the streamsfor a mill having a recovery boiler.

[0073] The utilisation of energy for the three presented process systemsis compared in Table 1 below, where the state-of-the-art pulp and papermill with a recovery boiler, is used as reference level. For the twoother process systems, the BLGCC and the BLGSF Table 1 shows thealteration in energy fed to or taken out from the two alternativescompared to the state-of-the-art reference system. TABLE 1 Comparison*⁾of utilisable energy for BLGCC and BLGSF systems Export of valuableEfficiency: Intake of extra Energy, MW MW(prod.) biomass, MW (stream 15)MW(feed) Pulp and Paper Mill — — combined with recovery boiler(reference system) Pulp and Paper Mill +51 +35 (power, 0.68 combined(stream 33) stream 34) with BLGCC Pulp and Paper Mill +210 +141(methanol, 0.67 combined with BLGSF (stream 33 + stream 15) 39)

[0074] The presented example produces methanol from biomass with anenergy efficiency of 67%, which is at least 15 percentage units higherthan state-of-the-art technology and on the same level as the mostenergy efficient methanol technology existing today, namely methanolproduction from natural gas. The example is based on a conventionalpower boiler 30 with moderate performance for conversion of biomass tosteam and further into electric power. If this boiler instead would beusing high performance data the energy efficiency would approach 80%. Inthe example, the additional required electric power is produced at thesame location in an advanced biomass fed IGCC power unit 37 to allow forcomparison between the three process systems on equal basis. This powercould as well be produced elsewhere. In such a case the bleed out gas 16will be used in the power boiler 30 or in other energy consumers withinthe system. It is also possible to produce the required additionalelectric power in an enlarged biomass boiler 30 thus eliminating thebiomass fed IGCC unit 37.

[0075] To clarify Table 1, FIGS. 5 and 6 explain the overall energyflows to and from the state-of-the-art configuration 44 and thealternate BLGSF configuration 45 respectively. The dotted linerepresents the configuration boundary. In FIG. 5 streams 19, 20 and 21together represent the biomass feedstock to the configuration and stream22 the product. There can be an import or an export of electric powerto/from the state-of-the-art configuration, FIG. 5. This is not part ofthe comparison and is therefore not shown in the figure as this onlyaccounts for the changes in energy flows while going fromstate-of-the-art technology to the configuration representing theinvention.

[0076] In FIG. 6 streams 19, 20, 21 and 22 are the same as in FIG. 5.When producing methanol 15 according to the invention, additionalbiomass 33 and 39 is required. The biomass is used to produce additionalheat and electric power in units 30 and 37 to such a level that thealternate configuraton 45 has the same degree of independence of importof fuel and power supply from its surroundings as for thestate-of-the-art configuration shown in FIG. 5. In the calculated caseas per Table 1 the energy in the methanol stream 15 represents 67% ofthe energy brought to the configuration in streams 33 and 39. With amore efficient power boiler 30 than used in the presented example theenergy efficiency can approach 80%.

[0077] Development work during the last decade in the field of replacingthe recovery boiler has as previously described focused on the BLGCCconcept. In most proposed concepts air has been used as oxidant in thegasifier resulting in the production of a diluted gas with a highconcentration of nitrogen coming from the air. Lately there has been ashift in focus to instead use oxygen as this leads to a number ofbenefits.

[0078] One benefit with the use of pure oxygen is that the produced gashas such properties that it with reasonable means can be converted Intoa synthesis gas for chemical synthesis. The quality of the gas differssignificantly from that normally produced from a gasifier fed with solidbiomass material and using oxygen as oxidant. Gasification of solidbiomass in the form of chopped pieces of wood leads to excessiveformation of methane and other higher hydrocarbons as previouslymentioned in the section describing state-of-the-art methanol productionfrom biomass. It can therefore be considered as a waste of a highquality intermediate process stream to just burn the synthesis gas fromblack liquor gasification in a gas turbine instead of using it as a highvalue feedstock to a chemical synthesis such as methanol, DME, hydrogengas, ammonia and others.

[0079] The presented embodiment thus reveals a biomass feedstockupgrading scheme where the energy-rich black liquor stream is used as avaluable resource for high quality synthesis gas. The energy, which isconverted to methanol and therefore not used as energy source for themill process, is thus replaced by energy from low quality biomassfeedstock fed to a standard power boiler and a biomass fed IGCC unit.

[0080] The preferred embodiment is further described with reference toFIG. 4. After withdrawal of green liquor 24 from the gasifier step 3 theuntreated synthesis gas 11 is cooled in the gas cooling step 4 beforefurther treatment. The present invention includes such advanced gaspurification where the untreated synthesis gas 12 is cooled down to lowtemperatures, preferably below −40° C., before it is cleaned by washingwith cooled methanol. This type of treatment has the advantage that ithas the capability to separate out undesirable higher hydrocarbonsexcluding methane.

[0081] The proposed gas cleaning process 5 also has the capability toremove both hydrogen sulphide and carbonyl sulphide that are bothpresent in the gas from the gasifier down to very low levels, <0.1 ppm,and to remove carbon dioxide down to the required level of 2-3% byvolume in stream 13. Due to its ability to be very selective, thecleaning step 5 can recycle the sulphur components back to the millprocess in a highly concentrated stream 35 and also produce a streamrich in carbon dioxide 41. Carbon dioxide may be useful within the millprocess 28 as shown with stream 43 e.g. in the pulp bleaching section ofthe mill process. Carbon dioxide may also have a value as feedstock forthe production of pure carbon dioxide for export while excess quantities42 will be emitted to the atmosphere.

[0082] The sulphur containing stream 35 may also be converted toelemental sulphur e.g. in a so-called Claus process before the sulphuris recycled to the mill process. The Claus process is normally part ofthe gas cleaning step 5. Selection of the preferred route is depends onthe management of sulphur within the pulp mill.

[0083] The selection of technology for gas cleaning 5 has an impact onoverall process reliability as well as on the ability of the BLGSFprocess to be a tool for mill process optimisation. The low operatingtemperature of the process and the high selectivity when removingsulphur components and carbon dioxide are key contributors.

[0084] In the gas-conditioning step 6 the ratio between carbon monoxideand hydrogen is adjusted to become 0.5 by mole fraction in stream 14.This is done by letting a part of stream 13 run through a so-calledshift reactor. In such a reactor water and carbon monoxide react tohydrogen and carbon dioxide over a catalyst under heat release. Afterthe shift reactor the shifted gas needs to be purified from the producedcarbon dioxide before said shifted stream is combined with thenon-shifted stream to form the methanol feed stream 14.

[0085] An alternative route is to put the gas conditioning step 6 beforethe gas cleaning step 5 to avoid a second cleaning of the shifted streamas previously described. The preferred embodiment is however to put thegas conditioning step as described in FIG. 4.

[0086] Clean synthesis gas adjusted for methanol production is fed tothe methanol synthesis in stream 14. To reach optimum conditions formethanol generation the pressure of the synthesis need to be at 60-80bar. According to the present invention gasification 3 is preferablytaking place at approximately 30 bar and therefore further compressionis preferably before the methanol synthesis 7. Gasification pressure canalso be selected to be higher to avoid an extra compression step orlower due to other process considerations.

[0087] The methanol synthesis step 7 consists of a loop wherenon-reacted gas is recycled and mixed with fresh gas from step 6. Thedegree of recycle is depends on the amount of inert molecules in thefeed and in the loop. Inert gas refers to those species notparticipating in the methanol formation reactions. Inert molecules aree.g. nitrogen and argon and partly methane. Carbon dioxide isparticipating in the reactions and its concentration needs also to bekept under control by bleeding out a part-stream of the recycle. Lessinert gas in the feed leads to less bleed-out and therefore to amaximised methanol yield. The quality of the gas from the gasificationstep therefore plays a key role to accomplish high yield. The methanolstream 15 is of quality called “topped” which generally meansapproximately 97-98% purity and that can be used as a low additive togasoline. If a 100% pure methanol is desired a distillation unit can beadded for a complete removal of water.

[0088] Although the invention has been described with regard to itspreferred embodiments, which constitute the best mode presently known tothe inventors, it should be understood that various changes andmodifications as would be obvious to one having the ordinary skill inthis art may be made without departing from the scope of the inventionas set forth in the claims appended hereto.

1. A process (44) for the production of pulp and paper (28), recyclingof cooking chemicals (3, 29), combustion of biomass (30, 37) andgeneration of heat and electric energy (27, 40) comprising a pulp andpaper mill (28), characterized in that the part of the process which isrecycling cooking chemicals is adjusted from combustion (29) togasification (3) to generate synthesis gas (14); and that biomass isadded (33, 39) in an amount sufficient for compensating for the decreasein heat and electricity generation as a consequence of the generation ofsynthesis gas (14).
 2. A process according to claim 1, characterized inthat the synthesis gas is methanol synthesis gas.
 3. A process accordingto claim 1, characterized in that the synthesis gas is dimethyle ether(DME) synthesis gas.
 4. A process according to claim 1, characterized inthat the synthesis gas is hydrogen gas.
 5. A process according to claim1 and 2, characterized in that said synthesis gas (14) is converted tomethanol, comprising combinations of the following processes, in whichthe second process below (29) is not directly involved: (28) A firstprocess for conversion of wood to produce pulp utilising cookingchemicals containing sodium and sulphur based salts and alsoco-producing a biomass derived, energy rich stream containing spentcooking chemicals (29) A second process for conversion of the energy insaid stream to usable energy for the first process (28) and recycle ofsaid cooking chemicals to the first process (28); (38) A third processfor conversion of the energy in said stream to methanol and usableenergy for the first process (28) and recycle of said spent cookingchemicals to the first process (28); (30) A fourth process forconversion of biomass derived material to heat and (37) A fifth processfor conversion of biomass derived material to electric energy; and wherein an original configuration (44) comprising the first, second andfourth processes (28), (29) and (30), the energy required for operationof said original configuration is partly brought to the configurationfrom the second process (29) where said energy rich stream from thefirst process (28) is converted to heat and electric energy and partlyby conversion of said biomass derived material (20,21) brought to thefourth process (30) where said material is converted to heat andelectric energy for said original configuration, in that, when thesecond process (29) is replaced by the third process (38) to become analternate configuration (45) and energy in said spent cooking chemicalsfrom the first process therefore in part is converted to methanol andwithdrawn from said alternate configuration, additional biomass derivedenergy (33, 39) is brought to said alternate configuration after beingconverted to heat and electric energy in the fourth process (30) and toelectric energy in the fifth process (37) so that the total requirementof heat and electric energy for said alternate configuration (45) is metat an equal level as in said original configuration (44).
 6. A processaccording to claim 5 characterised in that energy withdrawn from saidalternate configuration (45) in the form of methanol corresponds to atleast 60% of the energy contained in said additional biomass (33, 39)brought to said alternate configuration (45) to compensate for thewithdrawal of said methanol (15).
 7. A process according to claim 5,characterized in that the chemical produced from the third process (38)is DME
 8. A process according to claim 7 characterised in that energywithdrawn from said alternate configuration (45) in the form of DMEcorresponds to at least 60% of the energy contained in said additionalbiomass (33, 39) brought to said alternate configuration to compensatefor the withdrawal of said DME.
 9. A process according to claim 5,characterized in that the chemical produced from the third process ishydrogen gas
 10. A process according to claim 9 characterised in thatenergy withdrawn from said alternate configuration (45) in the form ofhydrogen corresponds to at least 60% of the energy contained in saidadditional biomass (33, 39) brought to said alternate configuration tocompensate for the withdrawal of said hydrogen.
 11. A process accordingto claim 5-10 characterised in that the fifth process (37) forconversion of biomass derived material to electric energy is located ata remote location from said alternate configuration (45) and that saidelectric energy is brought to the alternate configuration (45) via anelectric distribution grid.
 12. A process according to any of claims1-10, characterized in that sulphur components, such as sulphide andother sulphur components, are removed from the syntheses gas (14)preferably to a concentration below about 0.1 ppm, and recycled to themill process (28) in a highly concentrated stream (35).